1. Field of the Invention
This invention relates to a throttle valve control system for an internal combustion engine, and more particularly to a system for electrically controlling the throttle valve in response to operation of the accelerator.
2. Description of the Prior Art
In U.S. Pat. No. 4,112,885, there is disclosed an electric control type throttle valve control system in which movement of the accelerator is converted to an electric signal and a driving motor for driving the throttle valve is energized according to the electric signal to open and close the throttle valve in response to the movement of the accelerator. This system is advantageous over conventional throttle valve control systems, in which the throttle valve and the accelerator are mechanically connected with each other by a link mechanism, a wire mechanism or the like, in that the throttle valve can be relatively freely controlled to obtain a desired engine output, and that the accelerator depression effort can be minimized.
However, the conventional electric control type throttle valve control system is disadvantageous in that the amount of intake air does not linearly change with respect to the amount of depression of the accelerator pedal since the throttle valve and the accelerator are operatively connected so that the opening degree of the throttle valve linearly changes with respect to the amount of operation of the accelerator as in the conventional mechanical throttle valve control system and the change in the effective opening area of the intake passage for a given change in the opening degree of the throttle valve differs with the opening degree of the throttle valve. Therefore, during cruising in which the amount of operation of the accelerator is intermediate, a slight change in the amount of operation of the accelerator reduces or increases intake air by a large amount so as to adversely affect stability during cruising. On the other hand, when the accelerator is further pressed down during travel at a wide throttle to accelerate the vehicle, the amount of intake air is hardly increased and accordingly, the vehicle cannot be satisfactorily accelerated. Further, since the amount of intake air hardly changes with change in the amount of operation of the accelerator when the opening degree of the throttle valve is low, response of the engine is slow upon starting.